A method for suppressing the growth of mycobacteria associated with hypersensitivity pneumonitis, in particular Mycobacterium immunogenum, in metalworking fluids is described. The method uses antimicrobial or biocidal compositions which include a metal complex comprising disodium monocopper (II) citrate and an isothiazolone mixture preferably comprising 5-chloro-2-methyl-4-isothiazolin-3-one 2-methyl-4-isothiazolin-3-one. Also described are methods for detecting and identifying the mycobacteria.